In energy plants of the above kind the walls of the bed vessel are subjected to heavy forces because of the pressure difference between the inside and the outside of the bed vessel. In a PFBC energy plant with the bed vessel enclosed in a pressure vessel and being surrounded by compressed combustion air, a pressure difference between the space in the pressure vessel outside the bed vessel and the space inside the bed vessel arises because of the pressure drop in inlet channels and nozzles for the supply of air for fludization of a bed material in the lower part of the bed vessel and a pressure drop in the fluidized bed. This pressure difference may amount to about 0.1 MPa. The side walls of the bed vessel may have the size 10.times.15 m, so the forces acting on the bed vessel walls will be very great. This, in addition to a high temperature, entails design problems which are difficult to deal with.
The walls of the bed vessel consist of panels of tubes which are connected to intermediate fins. These walls, often called panel walls, may be cooled by feedwater circulating in the tubes. The panel walls are incapable of absorbing the loads caused by the pressure difference between the two sides of the walls. The bed vessel is therefore surrounded by a force-absorbing frame structure. The bed vessel is connected to this frame structure by means of force-transmitting bars or links. In case of a cold plant, the frame structure and the bed vessel have the same temperature. In operation, the bed vessel wall assumes the temperature of the circulating coolant and the frame structure the temperature of the surrounding air. Depending on temperature differences between the bed vessel wall and the force-absorbing frame structure, the bed vessel may expand or contract in relation to the frame structure.
The connection between the frame structure and the bed vessel must be designed in such a way that the difference in expansion does not give rise to impermissible stresses in the bed vessel, the frame structure or the connecting members between these.
German Offenlegungsschrift 2 055 803 shows one way of carrying out the connection between a conventional boiler and a force-absorbing frame.
Another known design already occurs in the PFBC energy plants existing at the Varta plant in Stockholm and at Escatron in Spain. In these plants, a stiffening of the panel walls has been obtained by means of continuous support frames with stiff corners extending horizontally around the bed vessel. These frames have been made in the form of box girders welded-together at the corners and they have been given the ability to absorb thermal movements in the bed vessel, among other things by means of an arrangement with auxiliary beams in the corners of the bed vessel, as shown in European patent application 87117795.2.
The factors which must be taken into consideration when dimensioning beams in a frame construction of the kind mentioned are, among other things, horizontal bending stress caused by forces due to pressure difference along the beam from the panel wall stiffened by the beam, vertical bending stress caused by attached equipment, torsion due to uneven load from vertical auxiliary members which in the upper and lower frames are connected between the frame and the upper edge and lower edge, respectively, of the bed vessel wall, the risk of breaking due to bending in the vertical direction of the beams caused by great axial compressive forces, twisting of the beams, transverse forces, combined stresses and, finally, fatigue conditions.
The above-mentioned box girder design for frames have been chosen because it should withstand all of the different stresses enumerated above. However, a frame structure with box girders has proved to be heavy and material-demanding. In addition, it requires a considerable effort in the welding work to form the box girders in accordance with the requirements.
A basic design with continuous frames, provided with stiff corners, around the bed vessel is desirable in order to reduce deflections and stresses. Conventional solutions with beams which are freely mounted at the ends of the bed vessel corners are not considered to fulfil the demands imposed, among other things because these solutions give higher maximum moments on the beams. On the other hand, a possibility of utilizing standard beam sections in a frame structure with continuous frames with stiff corners is preferred, in order to considerably reduce the weight and reduce the work demanded during manufacturing, which renders the entire design simpler and less expensive. The types of load which particularly must be accounted for when changing from box girders to standard beam sections in a support frame according to the above are primarily breaking due to bending, the risk of twisting, and torsion caused by connected auxiliary beams. The present invention presents a solution to the problems described above.